The Earth is not
the fixed, solid mass that we usually envision. It is actually a sphere of
solids and molten rock fluids that are gradually but continuously moving and
changing. For example, South America is drifting away from Africa at about the
speed your fingernails grow. Earthquakes and volcanoes are reminders of the
Earth's instability and changing face.

The planet's
crust is divided into numerous tectonic plates. These push against each other,
rise and fall, tilt and slide, buckle and crumple, break apart and merge
together. As a result, sediments from the bottom of ancient seas can today be
found in rocks on the tops of mountains. In fact, the summit of Mount Everest is
marine limestone, formed just this way.

For more than
half a billion years, photosynthesis has made life possible on Earth. Plants
absorb solar energy and use it to convert carbon dioxide and water into oxygen
and carbohydrates, such as sugar, starch and cellulose. These carbohydrates and
other organic materials eventually settle on the ground and in stream, lake and
sea beds.

As these organic
materials become more deeply buried, heat and pressure transform them into
solid, liquid or gaseous hydrocarbons known as fossil fuelscoal, crude oil or
natural gas. Coal is generally formed from the remains of terrestrial
(land-based) plants. Oil is typically derived from marine (water-based) plants
and animals, mainly algae, that have been gently "cooked" for at least
one million years at a temperature between 50° and 150° Celsius. Natural gas
can be formed from almost any marine or terrestrial organic materials, under a
wide variety of temperatures and pressures.

Due to the force
of gravity and the pressure created by the overlying rock layers, oil and
natural gas seldom stay in the source rock in which they are formed. Instead,
they move through the underground layers of sedimentary rocks until they either
escape at the surface or are trapped by a barrier of less permeable rock.

Most of the
world's petroleum has been found trapped in porous rocks under relatively
impermeable formations. These reservoirs are often long distances away from the
original source.

A seep occurs
when hydrocarbons migrate to the Earth's surface. Over time, huge amounts of
these hydrocarbons have escaped into the atmosphere. Flowing water can also wash
away hydrocarbons. Sometimes only the lighter, more volatile compounds are
removed, leaving behind reservoirs of heavier types of crude oil.

The Athabasca oil sands in northeastern Alberta are one example of a petroleum resource that
has lost its lighter components or fractions. The tar-like bitumen in the oil
sands was formed largely by the effects of bacterial processes, water flows and
oxidation on the petroleum in the reservoir.